As previously reported, myofibrillar ATPase activity decreases with age while the duration of the isometric twitch in papillary muscles increases and do not appear to be related. Myosin ATPase activity and the percent of myosin isoenzyme V1 have also been shown to decrease with age as well as with pathologic hypertrophy. Animals treated with thyroxine increased myosin ATPase activity and decreased the contraction duration and have suggested that the two parameters are related. Therefore, we treated young (2 mo), adult (8 mo), and senescent (24 mo) rats with thyroxine to produce a hyperthyroid state and were able to show that the isometric contraction time parameters decreased without altering the maximum myofibrillar ATPase activity in any group. However, when the myosin isoenzymes V1 and V3 were examined, a redistribution of these isoenzymes in the euthyroid state occurred with age and that was reversed by thyroxine treatment. Similarly, Ca2+-myosin ATPase activity in euthyroid animals showed a linear decrease with age that was only reversible in the 24 month animal where the percent of myosin isoenzyme V1 and ATPase activity were severely depressed. Therefore, we conclude that age associated decrease in the Ca2+-myosin ATPase activity, myosin isoenzyme distribution, and prolongation of isometric contraction time parameters is not fixed and can be reversed with thyroxine treatment. These changes were present even though T4 treatment caused hypertrophy of both sides of the heart. This suggests that while the myosin isoenzymes and ATPase activity may play an important role in myocardial contraction, other functions of the cell, such as sarcoplasmic reticulum Ca2+ sequestration, also have an important role in excitation-contraction coupling. The results also suggest that ventricular hypertrophy may be only a response, and not a cause, of alterations in myocardial biochemistry and contractile activity.